Conventionally, a liquid crystal display (LCD) device using a nematic-type liquid crystal display element has been widely used as a digit-segment-type liquid crystal display device for use in a watch or a electric calculator. Recently it has come to be used as a display device for use in a word processor, a computer, a navigation system, or the like.
Among such LCD devices, in particular, an active matrix LCD device wherein active elements such as TFTs are used as switching elements and pixels are arranged in a matrix form is widely used and known. The LCD device has advantages such as having a drastically reduced thickness (depth), consuming less power, and being easily modified to a full-color display device, as compared with the CRT display device. Therefore, the demand for the LCD device is increasing in various fields such as those of personal computers, various monitors, potable TVs, and cameras. However, since such a conventional LCD device is inferior to the CRT display device in a viewing angle range, luminance, color reproduction, and the like, improvement is eagerly desired in respect to these aspects.
The active matrix LCD device has a transparent active matrix substrate, on which a plurality of pixel electrodes 51 for applying voltages to a liquid crystal layer are arranged in a matrix form as shown in FIG. 11. As active elements which are switching means for selectively driving the pixel electrodes 51, thin-film transistors (TFTs) 52 are formed on the substrate and are connected with the pixel electrodes 51. Further, in the case where the color display is conducted, color filter layers of red, green, blue, and other colors are provided, though not shown, on the active matrix substrate or a counter substrate, in addition to the foregoing arrangement.
Gate electrodes of the TFTs 52 are connected with scanning lines 53, while source electrodes of the TFTs 52 are connected with signal lines 54. The scanning lines 53 and the signal lines 54 are provided so as to run beside the pixel electrodes 51 arranged in matrix and orthogonally cross each other. The TFTs 52 are driven in response to input of gate signals through the scanning lines 53. Upon the driving of the TFTs 52, data signals (display signals) are supplied to the pixel electrodes 51 through the signals lines 54 and the TFTs 52.
Furthermore, drain electrodes of the TFTs 52 are connected with the pixel electrodes 51 and additional capacitors 55. Counter electrodes provided vis-a-vis the additional capacitors 55 with an insulating layer therebetween are connected with common lines 56. The additional capacitors 55 are intended to hold voltages to be applied to the liquid crystal layer.
In the active matrix LCD device, the liquid crystal layer is provided between the active matrix substrate and the counter substrate vis-a-vis to the active matrix substrate. In other words, the liquid crystal layer is provided between the pixel electrodes 51 on the active matrix substrate and the counter electrodes provided on the counter substrate, thereby constituting a liquid crystal capacitor. The additional capacitors 55 are connected in parallel with the liquid crystal capacitor.
To explain each TFT 52 in more detail, as shown in FIG. 12, a gate electrode 62 is formed on a transparent insulating substrate 61, and a gate insulating film 63 is formed so as to cover the gate electrode 62. On the gate electrode 62, a semiconductor film 64 is provided with the gate insulating film 63 interlaminated therebetween. On a center portion of the semiconductor film 64, there is provided a channel protective layer 65. On source section sides of the channel protective layer 65 and the semiconductor thin film 64, a source electrode 66a composed of a microcrystal n.sup.+ -silicon layer is formed, while on drain section sides of the same, a drain electrode 66b composed of a microcrystal n.sup.+ -silicon layer is formed.
The source electrode 66a is connected with a metal layer 67a serving as a source wire, while the drain electrode 66b is connected with a metal layer 67b serving as a drain wire. A surface of the TFT 52 is covered with an interlayer insulating film 68, and a transparent conductive film serving as the pixel electrode 51 is formed thereon. The pixel electrode 51 is connected with the metal layer 67b as the drain wire of the TFT 52 through a contact hole 69. Further, on the pixel electrode 51, an alignment film (not shown) for aligning the liquid crystal is uniformly provided substantially throughout a whole display region including marginal portions.
As described above, the interlayer insulating film 68 is provided between the scanning lines 53 and the signal lines 54 on one hand and the transparent conductive films serving as the pixel electrodes 51 on the other hand. Therefore, the pixel electrodes 51 can be laminated on the scanning lines 53 and the signal lines 54 with the interlayer insulating film 68 interlaminated therebetween. Such arrangement is disclosed by, for example, the Japanese Publication for Laid-Open Patent Application No.58-172685/1983 (Tokukaisho 58-172685). With the arrangement, a pixel aperture ratio of the LCD device is enhanced, and disclination of the liquid crystal is suppressed by shielding an electric field due to signals conducted through the signal lines 54 with the use of the interlayer insulating film 68.
As the interlayer insulating film 68, an inorganic thin film made of SiN or the like has conventionally been used. The SiN film is formed by, for example, the CVD (chemical vapor deposition) method to a thickness of about 500 nm.
Here, liquid crystal molecules used as the liquid crystal layer have a refractive index anisotropy .DELTA.n, and the liquid crystal molecules are aligned with an inclination with respect to the active matrix substrate and the counter substrate sandwiching the liquid crystal molecules. Therefore, a contrast of a displayed image alters depending on a viewing direction or a viewing angle of an observer, whereby aggravating the viewing angle dependence.
Regarding the foregoing problem, a liquid crystal display method for an LCD device of the twisted-nematic (hereinafter referred to as TN) type, which is particularly often used among the LCD devices of the nematic type, is explained as follows. As shown in FIG. 13, when a voltage for a half-tone display is applied to an LCD element 71 of the TN type, each liquid crystal molecule 72 slightly raises an end thereof. Here, a linearly polarized light 75 running in a normal direction of surfaces of substrates 73 and 74, and linearly polarized lights 76 and 77 running in a direction inclining with respect to the normal direction cross the liquid crystal molecules 72 at different angles. Since the liquid crystal molecules 72 have the refractive index anisotropy .DELTA.n as described above, an ordinary light and an extraordinary light occur when the linearly polarized lights 75, 76, and 77 directed in the respective directions pass through the liquid crystal molecules 72. T he linearly polarized lights 75, 76, and 77 are converted to elliptically polarized lights in accordance with phase differences between the ordinary light and the extraordinary light, respectively, thereby causing the viewing angle dependence.
Inside the liquid crystal layer, among the liquid crystal molecules 72, those around a mid point between the substrates 73 and 74, those near the substrate 73, and those near the substrate 74 have different tilt angles. Besides, those near the substrate 73 are twisted with respect to those near the substrate 74 through an angle of 90.degree. a round an axis directed in the normal direction. For these reasons, when passing through the liquid crystal layer, the linearly polarized lights 75, 76, and 77 are affected by various birefringence effects depending on the directions and angles thereof, thereby exhibiting complicated viewing angle dependence.
As concrete phenomena of the viewing angle dependence, the following phenomena occurs, in the case where an LCD device is disposed so that better visibility is obtained when viewed .backslash.in a direction inclined toward a top side.backslash. of the screen, as shown in FIG. 14: (1) as the viewing direction is inclined from the normal direction of the screen to a normal viewing angle direction which is a direction inclined to a bottom side of the display screen, the display becomes discolored (hereinafter referred to as discoloring) when the viewing angle exceeds a certain degree, or black and white reverse (hereinafter referred to as reversal); and (2) as the viewing direction is inclined to an anti-viewing angle direction which is a direction inclined to a top side of the screen, the contrast drastically deteriorates.
On top of that, the aforementioned LCD device has a drawback in that the angle of visibility narrows as the display screen expands. In viewing a large LCD screen at a short distance from the front, colors displayed in an upper part and in a lower part sometimes differ due to an influence of the viewing angle dependence. This is because an angle of vision for viewing the whole screen becomes greater, whereby viewing an edge portion of the large screen becomes identical to viewing a smaller LCD screen in a direction further inclined.
To suppress the viewing angle dependence, to insert a phase difference plate (phase difference film) as an optical element having an optical anisotropy between the LCD element and one polarizing plate has been proposed (see, for example, the Japanese Publication for Laid-Open Patent Application No.55-600/1980 (Tokukaisho 55-600)).
According to this method, light converted from the linearly polarized light to the elliptically polarized light as described above is caused to pass through the phase different plate(s) provided on one side or both the sides of the liquid crystal layer, so as to be again converted to linearly polarized light, whereby a phase difference between ordinary light and extraordinary light occurring to the viewing angle is compensated, and suppression of the viewing angle dependence is enabled. Therefore, to conduct this method, it is necessary to adjust not only properties of the phase difference plate but also those of the liquid crystal layer, i.e., the LCD element.
Then, to further suppress the viewing angle dependence, an LCD device arranged as follows is proposed (see the Japanese Publication for Laid-Open Patent Application No.5-313159/1993 (Tokukaihei 5-313159)): the LCD device has (i) an LCD element which is arranged so that a retardation .DELTA.n.multidot.d which is a product of a refractive index anisotropy .DELTA.n of a liquid-crystalline material of the liquid crystal layer and a thickness d of the liquid crystal layer is in a range between 200 nm to 500 nm and (ii) a phase difference plate provided between the LCD element and a polarizing plate, the phase difference plate being arranged so that a main refractive index direction of its index ellipsoid is parallel with a normal direction of a surface of the phase difference plate.
This arrangement is characterized in that the properties of the phase difference plate and the LCD element are desirably set not only as described above but also so that a rubbing direction of an alignment film constituting the LCD element, a slow axis direction of the phase difference plate, and a transmission axis of the polarizing plate are parallel, thereby ensuring further suppression of the viewing angle dependence.
Even in the case where such a phase difference plate is used, reversal which occurs when viewed in the normal viewing angle direction is still recognized in a narrow angle range.
Furthermore, to use another arrangement wherein a main refractive index direction of the index ellipsoid is inclined with respect to the normal direction of the surface of the phase difference plate has been also proposed (see the Japanese Publication for Laid-Open Patent Application No.6-75116/1994 (Tokukaihei 6-75116)). In this arrangement, either of phase difference plates of the following two types is used.
One is arranged so that among three main refractive indexes of the index ellipsoid, the smallest main refractive index has a direction parallel with the surface of the phase difference plate, and one of the rest two main refractive indexes is inclined at an angle .theta. with respect to the surface while the other is inclined at an angle .theta. with respect to a normal direction of the surface of the phase difference plate, where .theta. satisfies 20.degree..ltoreq..theta..ltoreq.70 .degree..
The other is arranged so that: (1) the phase difference plate does not have a refractive index anisotropy around its surface; (2) a main refractive index nb in a normal direction of the surface of the phase difference plate and main refractive indexes na and nc parallel with the surface of the phase difference plate satisfy na=nc&gt;nb, that is, the phase difference plate is negatively uniaxial; and (3) the index ellipsoid is inclined by rotating the main refraction index nb direction in a clockwise direction or in an anti-clockwise direction around an axis which is either of the directions of the main refractive indexes na or nc so that the index ellipsoid shifts from a state of being parallel with the normal direction of the phase difference plate surface to a state of being inclined to the same.
Regarding the foregoing two types of phase difference plates, the former may be uniaxial or biaxial. As to the latter, instead of using a single phase difference plate, a combination of two phase difference plates whose main refractive index nb directions resulting on the inclination described above have an angle of 90.degree. therebetween may be used.
In the LCD device thus arranged so as to have at least one such phase difference plate between the LCD element and the polarizing plate, the viewing angle dependence is suppressed to some extent. As shown in FIG. 15 which illustrates an example of this, improvement regarding the contrast and the suppression of reversal is achieved in this case, as compared with the case of FIG. 14: the contrast is improved substantially in all directions, and reversal in the normal viewing angle direction is also further suppressed, not occurring till the viewing angle exceeds about 350.
As another means to suppress the viewing angle dependence, an arrangement wherein regions differing in the tilt angle and the alignment are formed in one pixel electrode region has been also proposed (see the Japanese Publications for Laid-Open Patent Applications No.5-210099/1993 (Tokukaihei 5-210099) and No.7-64096/1995 (Tokukaihei 7-64096)). With this arrangement, the viewing angle dependence can be suppressed to some extent.
Furthermore, a new-type LCD device which utilizes a so-called IPS (in-plane switching) scheme has been developed as an LCD device of a new arrangement, and mass-production of the same has been promoted (see Nikkei Microdevices, July 1997, pp.108-110). This is the display scheme which provides the most excellent viewing angle properties among various display schemes now being subject to practical application, and this is actually installed in monitors of computers and the like.
However, in the case of the aforementioned arrangement disclosed by Tokukaisho 58-172685 wherein SiN.sub.X, SiO.sub.2, TaO.sub.X, or the like is deposited by the CVD method or the sputtering method so as to form a transparent insulating film which is served as the interlayer insulating film 68, as shown in FIG. 12, a surface of the interlayer insulating film 68 becomes uneven due to unevenness of the surfaces of the metal layers 67a and 67b, the channel protective layer 65, and the like thereunder. Therefore, in the case where the pixel electrodes 51 are formed on the interlayer insulating film 68, further greater level differences occur in accordance with level differences of various films underneath, whereby defects in liquid crystal molecule alignment (disclination) occur.
Furthermore, in the case where, to flatten a surface on which a pixel section is formed, an organic film is formed by applying polyimide or the like conventionally available in the market, a step for forming contact holes for electrically connecting the pixel electrodes 51 with the drain electrodes 66b has to be added to the manufacturing process. More concretely, a step of conducting photo-patterning with respect to the organic film made of polyimide or the like by using a mask, then, forming contact holes by etching, and finally, peeling off a photoresist which is no longer necessary has to be added.
Besides, if such organic film conventionally sold in the market is applied to the LCD device arranged as described above, the resin appears discolored after the interlayer insulating film 68 is formed.
For these reasons, such arrangement does not render the LCD device high translucency, transparency, and color reproductivity.
A photosensitive polyimide film may be used as the interlayer insulating film 68 to cut the etching and peeling step, but if a conventional material is used, it appears more discolored, resulting in further deterioration of the display quality. For this reason, this arrangement cannot be applied to an LCD device required of high translucency and transparency.
Furthermore, in the case where, as in the aforementioned arrangement, the pixel electrodes 51 are allowed to be laminated above the scanning lines 53 and the signal lines 54 by forming the interlayer insulating film 68 therebetween so that the aperture ratio of the LCD device is improved, electric capacity between the scanning lines 53 and the signal lines 54 on one hand and the pixel electrodes 51 on the other hand increases, thereby impairing the display quality.
More specifically, since an inorganic film such as an SiN film has a high dielectric constant such as 8 and such a film is formed by the CVD method or the like, it has to be formed thin to 500 nm or less with a view to reducing defects caused by film forming stress and film forming time (reflected in costs). For these reasons, the electric capacity between the scanning lines 53 and the signal lines 54 on one hand and the pixel electrodes 51 on the other hand drastically increases.
Therefore, the following problems (1) and (2) arise.
(1) In the case of an arrangement wherein the signal lines 54 and the pixel electrodes 51 are laminated, the signal transmittance (leakage of signals) increases due to an increase in the electric capacity between the signal lines 54 and the pixel electrodes 51. This causes the signal voltages held by the pixel electrodes 51 during a signal holding time to fluctuate depending on potentials of the data signals on the signal lines 54. This further causes effective values of the voltages (effective voltages) to alter upon application of voltages by the pixel electrodes 51 on the liquid crystal, thereby particularly causing vertical cross-talk to pixels neighboring in vertical directions in display actually obtained.
A driving method of an LCD device wherein each signal line 54 is supplied with a set of data signals corresponding to the signal line 54 with the polarity thereof alternately inverted has been proposed, for example, in the Japanese Publication for Laid-Open Patent Application No.6-230422/1994 (Tokukaihei 6-230422) so as to reduce the influence of electric capacity between the signal lines 54 and the pixel electrodes 51 onto the displayed images. This driving method is capable of rendering great effects to an LCD device wherein neighboring pixels have close correlation concerning display, as in a monochromatic LCD device.
However, in an LCD device wherein pixel electrodes are arranged in a vertical stripe form, as in a usual notebook-type personal computer, neighboring pixels connected with one signal line 54 have different colors to display. In the case of color display in particular, for example, in one pixel region shaped in a square, three pixel electrodes 51 corresponding to three colors of red (R), green (G), and blue (B) respectively are arranged in this order in a rectangular shape which is long in a vertical direction (this arrangement of the pixel electrodes 51 is hereinafter referred to as a vertical stripe arrangement).
In the case of a usual color display device having pixel electrodes arranged in such a vertical stripe form, a sufficient effect of suppressing vertical cross-talk cannot be achieved by the aforementioned driving method wherein the polarity is alternately inverted for each signal line 54.
(2) In the case where the scanning lines 53 and the pixel electrodes 51 are laminated, the electric capacity between the scanning lines 53 and the pixel electrodes 51 becomes greater. Therefore, a drawback in that a field-through of a write voltage applied to the pixel electrodes 51 becomes greater arises from a switching signal for controlling the TFTs 52.
Note that it is possible to suppress the increase in the electric capacity by forming the inorganic film to not less than 500 nm in film thickness. However, as the inorganic thin film is formed thicker, a time consumed for this step in the manufacturing process increases thereby raising the costs, while cracks tend to occur due to a film formation residual stress thereby causing an increase in defects and deterioration of reliability.
Besides, with the arrangement disclosed in Tokukaihei 5-313159, it is possible to suppress the viewing angle dependence of the display screen in a specific direction, but the arrangement is not sufficient to suppress the viewing angle dependence in all directions.
Furthermore, the arrangement disclosed by Tokukaihei 6-75116 is insufficient as well to solve the reversal, particularly, the reversal which occurs when viewed in the normal viewing angle direction. In the arrangement disclosed in Tokukaihei 6-75116, as described above, conditions of the phase difference plate are set so that the index ellipsoid is inclined. On the other hand, in an embodiment, used is an LCD element whose liquid-crystalline material has an refractive index anisotropy .DELTA.n of 0.08 and whose liquid crystal layer has a thickness d of 4.5 .mu.m, i.e., whose liquid crystal layer has a retardation .DELTA.n.multidot.d of 360 nm. However, nothing more than the above is mentioned about what type of an LCD element and what type of a polarizing plate should be used so that the phase difference plate is provided therebetween.
As for the arrangement wherein the viewing angle dependence is suppressed by providing the phase difference plate between the LCD element and the polarizing plate, it is necessary to set properties of not only the phase difference plate but also the LCD element. Therefore, regarding this arrangement, in what range the value of .DELTA.n.multidot.d of the LCD element combined with the phase difference plate should be in order to most efficiently conduct compensation of the phase difference is unclear. As a result, there is a drawback in that the arrangement is still insufficient to suppress the viewing angle dependence of the LCD device with the use of the phase difference plate.
Furthermore, in an arrangement disclosed in the Japanese Publication for Laid-Open Patent Application No.8-50206/1996 (Tokukaihei 8-50206) which is improved as compared with Tokukaihei 5-313159, the viewing angle properties are considerably improved in all directions as compared with the conventional phase difference plate, whereas it is difficult to say that the problem of the gradation reversal which occurs when viewed in the normal viewing angle direction is sufficiently solved.
Moreover, with an arrangement disclosed in the Japanese Publication for Laid-Open Patent Application No.7-64096/1995 (Tokukaihei 7-64096), which features division alignment, the viewing angle properties are equalized upward and downward, thereby remarkably solving the reversal when viewed in the normal viewing angle direction.
On the other hand, there is still a drawback in that the viewing angle properties rightward and leftward and in the anti-viewing angle direction are not sufficiently improved.
Besides, the LCD device of the IPS type is superior to any one of the aforementioned arrangements in the viewing angle properties in all directions. However, a pixel aperture ratio of the pixel section is low because of the comb-like electrode structure and the like. Therefore, in the case where it is adapted to be used in a monitor or the like, the number of light sources has to be increased to double of the usual number so that necessary luminance is obtained, thereby causing a weight and a size (particularly, a structural thickness) thereof to increase while consuming more power. Therefore, it is particularly unfavorable to use it in a notebook-type personal computer. Moreover, such a high-end notebook-type personal computer or a work station necessitates a transverse luminance of 200 cd/m.sup.2, and in order to obtain a traverse luminance comparable to that of a CRT display device, about 400 cd/m.sup.2 is necessitated. And in the case where a liquid crystal panel with a low pixel aperture ratio is further used, the number of light sources for a backlight has to be increased. Therefore, sometimes the liquid crystal has a great temperature rise, thereby becoming unusable, or requiring an extra cooling unit.